1. Field of the Invention
The present invention relates generally to a computer system having a PCMCIA (Personal Computer Memory Card International Association) slot in which a PCMCIA card is inserted, and specifically to a computer system employing the UMA (Unified Memory Architecture) technique that uses part of a work memory as a frame buffer (graphic memory).
2. Related Art
With the recent advancement in computer technology, computers have been utilized in various fields. In a car navigation system, for example, GPS (Global Positioning System) and computer technologies are utilized. The car navigation system displays a map and the current position of a vehicle on an LCD display and guides the vehicle to a destination, thereby allowing a driver to enjoy comfortable and safe driving without taking a wrong course in local areas of first visit.
FIG. 4 is a block diagram illustrating a configuration of a conventional car navigation system.
The car navigation system includes a CPU (Central Processing Unit) 41, a map data storage unit 42, a GPS receiver 43, a program memory 44, a work memory 45, a display control unit 46, a frame buffer 47, a monitor 48, and a PCMCIA slot 49. The CPU 41, map data storage unit 42, GPS receiver 43, program memory 44, work memory 45, display control unit 46, and PCMCIA slot 49 are mutually connected through a common bus 40.
The map data storage unit 42 comprises a CD-ROM or a DVD-M, on which is stored the map data, and a drive unit for the ROM. The GPS receiver 43 detects a current position (longitude and latitude) of the vehicle by means of a signal transmitted from the GPS satellite. The CPU 41 corrects the position detected by the GPS receiver 43 by using a signal outputted from a vehicle sensor (a sensor that detects the travel distance, travel speed, rotational angle, etc., which is not illustrated), and detects the current position of the vehicle with a higher accuracy.
The program memory 44 stores the programs for the navigation system to perform navigation (route guidance), and the CPU 41 executes various processing in accordance with the programs. The work memory 45 includes a DRAM that temporarily stores various data while the CPU 41 executes the programs.
The display control unit 46 generates images to be displayed on the monitor 48. For example, the CPU 41 reads map data of an area surrounding the current position of the vehicle from the map data storage unit 42, and transfers the data to the display control unit 46. The display control unit 46 generates image data (map image, etc.) in the frame buffer 47 on the basis of the map data transferred, and reads the image data from the frame buffer 47 at a specific timing to output to the monitor 48. Thus, the map image is displayed on the monitor 48.
In the PCMCIA slot 49 is inserted a card conforming to the PCMCIA standard (called the PCMCIA card), as needed. The PCMCIA card 50 can include a memory card, a modem card, etc. The memory card can record, for example, the data that illustrate a travel locus of a vehicle, the positions of sites that a user has voluntarily set, and the information (specific marks and comments, etc.) that are associated with the sites. Further, it is also possible to insert a memory card that bears an upgrade program in the PCMCIA slot 49 to thereby upgrade the program recorded in the program memory 44 and add a new function. Further, it is also possible to use the memory card inserted in the PCMCIA slot 49 as a work memory to thereby enhance the processing speed.
Further, in case a modem card is inserted in the PCMCIA slot 49, the modem card with a portable telephone connected thereto will allow a user to communicate with persons or facilities outside the vehicle. For example, it will become possible to connect with the Internet and transmit and receive mail, or to connect with a traffic information center and receive the most recent traffic information.
Currently, the memory capacity required for the frame buffer 47 is about 1 M to 2 M bytes, which is relatively low. However, the capacity of memory ICs being marketed has been increasing every year, and the memory IC of a low capacity necessary for the frame buffer 47 is becoming difficult to procure. If a large capacity memory IC is used for the frame buffer 47, it will be wasteful, and the cost per bit will become expensive. For example, if a 64 M bit DRAM is used, one memory IC will bear a memory capacity of 8 M bytes, and only about xc2xc of the memory capacity will be used.
Recently, a technique has been developed which uses part of the work memory as a frame buffer. This is the so-called UMA technique, which does not need to provide a memory IC exclusive for the frame buffer, and the use of the UMA technique can reduce the cost of a computer system accordingly. However, it is difficult to apply the UMA technique to a computer system that allows a connection of a PCMCIA card.
FIG. 5 is a timing chart illustrating the operation of a conventional computer system loaded with a PCMCIA card. In the PCMCIA standard, in the case of a CPU accessing a PCMCIA card, when the wait signal that the PCMCIA card outputs becomes active (xe2x80x9cHighxe2x80x9d in FIG. 5), the CPU needs to delay the termination of the memory access cycle by extending the bus cycle. The CPU, waiting for the wait signal to become non-active, reads data from the PCMCIA card. While the wait signal is active, the common bus is occupied by the PCMCIA card, and there inevitably occurs a waiting time of maximum 12 xcexcsec. Accordingly, it will become impossible to transfer image data between the frame buffer in the work memory and the display control unit.
Although it is conceivable to provide the display control unit with a buffer for storing data for more than 12 xcexcsec, it will result in an increased cost, which detracts from the advantage of employing the UMA technique.
Thus, it is an object of the invention to provide a computer system having a PCMCIA slot that does not create deficiencies of display even when part of a work memory is used as a frame buffer and permits a cost reduction thereof.
Further, it is another object of the invention to provide a car navigation system that has a PCMCIA slot to facilitate functional expansions and permits a cost reduction by employing the UMA technique.
In order to accomplish the foregoing objects, the invention provides a computer system comprising: an operational processing unit; a PCMCIA slot in which a PCMCIA card can be inserted; a PCMCIA control unit connected with the PCMCIA slot; a work memory in which data are stored in accordance with operations of the operational processing unit; a display control unit that uses part of the work memory as a graphic memory and generates image data; and a common bus that is commonly connected to the operational processing unit, the PCMCIA control unit, the work memory, and the display control unit. In this configuration, the PCMCIA card control unit comprises: an address latch unit that latches an address transferred through the common bus; a write data latch unit that latches write data transferred through the common bus; a re ad data latch unit that latches re ad data transferred from the PCMCIA card; and an operation state signal output unit that outputs a signal indicating an operation state of the PCMCIA card.
In the computer system according to the invention, the PCMCIA control unit is connected between the common bus and the PCMCIA slot, and the PCMCIA control unit latches addresses, read data, and write data. At the moment that the PCMCIA card completes the preparation for the data read or data write, the PCMCIA control unit executes the data read or data write to the PCMCIA card. Further, since the operation state signal output unit of the PCMCIA control unit outputs a signal that indicates the operation state of the PCMCIA card, the operational processing unit is able to detect whether a data write into the PCMCIA card or a data read from the PCMCIA card is completed or not, by checking the signal outputted from the operation state signal output unit.
Thus, in the computer system according to the invention, since the PCMCIA control unit latches the addresses, read data, and write data, the bus cycle is not needed to be extended, and the computer system is able to reduce the time for which the PCMCIA card occupies the common bus.
Therefore, even when part of the work memory is used as a graphic memory, it becomes possible to access the work memory for the image data at a specific timing. Thereby, a memory IC exclusive for the graphic memory becomes unnecessary, which reduces the cost of a device using a computer such as a car navigation system, etc.